1. Field of the Invention
The present invention relates to an optical disk having a reduced amount of foreign substance, an optical information-storage medium, and an injection-molding process and apparatus suitable for manufacturing the optical disk substrate.
2. Description of the Related Art
An optical information-storage medium, such as an audio-disk, a laser disk, an optical disk memory, and a magneto-optical disk, records and/or reads out information by utilizing laser beams. In the optical storage medium, a transparent substrate, i.e., an optical disk substrate, prepared from polycarbonate, polymethyl methacryrate, glass or the like, is used, and foreign substances, such as dust or carbonized material, contained in the optical disk substrate have a considerable affect on the reliability of a recording and/or a reading out of information.
In consideration of the above circumstances, the foreign substances contained in a raw material were filtered off, during a purification or granulation step, to reduce the amount thereof. For example, Japanese Unexamined Patent Publication (KOKAI) No. 61-90345 and No. 63-91231 disclose such a filtration method.
The above Japanese Unexamined Patent Publication (KOKAI) No. 61-90345 discloses that an amount of the foreign substances having a particle diameter of 0.5 .mu.m or more should be 1.times.10.sup.5 /g or less in the optical disk substrate, and to comply with the above requirement, the foreign substances contained in the raw material such as a monomer should be removed by distillation and/or filtration, the manufacturing apparatus must be kept clean, and further, an inclusion of the foreign substances during a manufacturing step should be prevented.
The above Japanese Unexamined Patent Publication (KOKAI) No. 63-91231 discloses that an amount of foreign substances having a particle diameter of 1 .mu.m or more should be 10000/g or less in the storage medium, and to comply with the above requirement, the foreign substances are removed by filtration of a solution in an organic solvent, and further, the storage medium is formed from a resin composition which has been passed through a sintered metallic filter, in a molten state, to remove the fine foreign substances.
Although the above patent publications mention that the number of foreign substances having a particle diameter of at least 0.5 .mu.m or 1 .mu.m per 1 g of the raw material affects the recording and/or reading-out characteristics, for example, an error ratio (i.e., bit error ratio: BER), this number is not always an appropriate indicator for an evaluation thereof.
The amount of foreign substances cannot be sufficiently reduced only by removing the foreign substances from the unmelted resin, and thus an inclusion of dust should be prevented during a pelletizing step, or a production or inclusion of carbonized material should be avoided during an extruding step. Reference is made to Japanese Unexamined Patent Publications (KOKAI) No. 58-126119 and No. 61-95914.
The above Japanese Unexamined Patent Publication (KOKAI) No. 58-126119 discloses that the optical disk substrate is molded from a pellet prepared by incorporating an antioxidizing agent into a polycarbonate flake, and pelletizing the mixture, but, the process disclosed in the Publication has disadvantages such that the melting by heating during the pelletizing step causes a thermal deterioration of the material, and the inclusion of foreign substances cannot be sufficiently avoided during the pelletizing step.
The above Japanese Unexamined Patent Publication (KOKAI) No. 61-95914 discloses that the optical disk substrate is manufactured by carrying out a suspension polymerization of methyl methacrylate (MMA), to produce a transparent acrylic resin beads, and then carrying out an injection molding of the beads without pelletizing. The process disclosed in the Publication, however, is applicable only to an acrylic resin having a low heat resistance, moisture resistance, and strength, and is not applicable to a polycarbonate having excellent properties as the material for an optical disk, because, in the case of the polycarbonate, the polymer beads cannot be produced due to a difference in the polymerization step.